1. Field of the Invention
The present invention is directed to an X-ray computer tomography apparatus of the type which is capable of producing an X-ray shadowgraph, in addition to a computed tomogram.
2. Description of the Prior Art
X-ray computed tomography apparatuses are known in which the X-ray source emits a fan-shaped X-ray beam that strikes a detector row constructed from a series of detector elements. For the production of computed tomograms, the measurement arrangement of the X-ray source and the detector rows is rotated around a system axis. From the detector output signals, produced in various projections, a computer calculates an image of the transirradiated region of the patient.
For the production of overview exposures, namely X-ray shadowgraphs, it is known from U.S. Pat. No. 4,477,922 to lock the measurement arrangement against rotation in such a computed tomography apparatus, and to carry out a relative movement between the measurement arrangement and the patient table, or at least the slab of the table, in the direction of the system axis, e.g. by displacing the patient table (or slab). The path of movement of the patient table thereby corresponds essentially to the length of the X-ray shadowgraph. Depending on the integration time of the data acquisition system, the data of a number of projections can be combined to form a line of the X-ray shadowgraph. In conventional systems, the resolution in the X-ray shadowgraph in the direction of the system axis is determined essentially by the slice collimation used for the exposure.
For the representation of the shadowgraph, the data recorded with the data acquisition system are A/D converted and transferred to a central processor. In the central processor, a normalization of the data dependent on the measurement system is conducted, the measured radiation intensities are converted to attenuation values and a sliding combination, dependent on the speed of the movement of the measurement system relative to the patient, of the projections (measured at fixed time intervals) of attenuation values is undertaken to form equidistant image lines of the shadowgraph. In a further step, the shadowgraph (obtained in a cylindrical projection) is reformated into a flat image, with pixels that are likewise equidistant in the transverse direction. A two-dimensional development of the shadowgraph, for increasing the image contrast, with simultaneous contour enhancement. follows the construction of the image. The two-dimensional filtering can take place either in a separate calculation step or on-line. corresponding to U.S. Pat. No. 5,315,628, i.e. in parallel with the data acquisition.
It is also known to expose an X-ray shadowgraph using a matrix detector, as described in U.S. Pat. No. 4,352,986, which is constructed from a series of parallel detector rows. If the matrix detector covers the entire region of examination, a relative motion between the measurement arrangement and the patient table can be avoided.